1. Field of the Invention
An improved system for recovering heat from combustion flue gas.
2. Description of the Prior Art
The ever-recurring energy crises and large increase in the cost of fuel oil and other fuels has necessitated more efficient and economical usage of fuels, and has mandated a maximization of the recovery of heat from combustion processes. Thus in recent years it has become economically attractive to maximize heat recovery even though higher capital costs for facilities and equipment are encountered.
The recovery of heat from flue gas from industrial furnaces has been practiced in large-scale installations for many years. Thus it is common practice in facilities where a fuel is burned in a furnace e.g. to generate steam for industrial usages, and especially in large scale steam electric power plants where high pressure steam is generated in boilers by vaporizing boiler feed or condensate water, and the steam is expanded in a steam turbine which drives an electrical generator, to install complex and costly waste heat boilers, and economizers which recover a final increment of heat from the combustion flue gas via the preheating of the boiler feed or condensate water prior to passing the water to the steam boiler.
Heretofore it has not been economical to recover heat from the flue gas generated in smaller furnaces, such as those found in private homes and apartment houses, because the capital cost was prohibitive relative to the heat savings realized. However, as mentioned supra, the ever-rising price of various fuels, and especially fuel oil and natural gas, has now made it economically feasible to contemplate capital investment for the recovery of heat from even small amounts of flue gas as generated at relatively low flow rates and temperatures in private dwellings and apartment houses.
It has been known in the prior art to recover heat from flue gas via indirect heat exchange with air, the heated air then being used to support combustion of the fuel such as gas, oil or coal in the furnace which generates the flue gas. It has also been known to accomplish the indirect heat exchange by alternately passing the hot flue gas and cold combustion air through a heat recuperator or regenerator, which is a vessel filled with brick checkerwork or the like. The hot flue gas heats the bricks and is cooled, and subsequently the combustion air cools the bricks and is heated. Such schemes are described on pages 9-48 through 9-51 of the 5th Edition of the Chemical Engineers' Handbook by Robert H. Perry and Cecil H. Chilton, published 1973 by McGraw-Hill Book Company.
Among the prior art relative to systems for recovering heat from hot flue gas by preheating ambient combustion air may be mentioned U.S. Pat. Nos. 3,207,493; 1,925,941; 1,835,210 and 1,505,767.